Recent image forming apparatus, such as an electrophotographic system, has high speed properties, multi-functions, and a network function, and enlarges their usages as a printing apparatus by connecting with a high-capacity sheet feed apparatus and a high-capacity stacker. In case the image forming apparatus is used as a printing apparatus, the image forming apparatus becomes possible to carry out from printing to bookbinding with one apparatus by connecting with a post-processing apparatus which binds printed matters (paper sheets on which images were formed).
As the post-processing apparatus, there has been known a gluing bookbinding apparatus, which is made small by arranging, devices, such as a sheet bundle storing section, a glue (an adhesive) application section, a cover sheet supply section, and a cover folding section in a column fashion in the vertical direction. The above bookbinding apparatus forms a booklet by applying an adhesive uniformly using an adhesive applying roller to a back of a sheet bundle, which is stored in a sheet bundle storing section and gripped. However, in case where an adhesive is applied only to a back of a sheet bundle, the bonded area is small, resulting in a shortage of strength of the glued binding. Coping with the above problem, it has been carried out to try to increase in the bonded strength by increasing a bonded area by forming a plurality of cut-outs using a perforation device at an end side of sheets, to which an adhesive is applied during a binding process of sheets.
Hereinafter, the above cut-out formation is also referred to as roughening.
The formation of a plurality of cut-outs using the above perforation device can be carried out while displacing perforation positions, even in case where, for example, the number of punch is one or two, which number is less than the number of cut-out. However, the punching while displacing perforation positions had a problem that it takes a lot of time for the perforation processing, resulting in decrease in processing ability of the whole apparatus. This problem affected, in particular, a high-speed machine.
Coping with the problem, there has been used a perforation device, which is provided with a plurality of punches, and forms a necessary number of cut-outs in one single action of perforation. However, in case where a plurality of punches are pushed down at the same time to punch holes in a sheet, a large force is required during the perforation. Further, there was a problem that, when the perforation is carried out at a time at an edge side of sheets, a force working in the direction of a sheet moving away increases, whereby a formation of cut-outs exhibiting an excellent shape and accuracy in position became difficult.
In order to minimize the above problem, it has been known that a differential perforation is effective, in which the perforation is successively carried out by allowing the perforation by each punch to have a time difference.
Corresponding to the above, a multi-hole punch (a perforation device) has been disclosed (refer, for example, to Patent Document 1) having a mechanism in such a manner that a lot of punches are arranged linearly, a punch holder (a holding board), which has a shape that the center part is convex downward and has a stairlike bumps in the both directions, is arranged in movable up and down, in the upper part of the above punches, and each of the punches comes down step by step in an order from the center one corresponding to a descent of the above punch holder.    Patent Document 1: Japanese Patent Application No. H10-15899